linux-kernel - Re: [PATCH] spi: Limit the spi device max speed to controller's max speed

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Date:   Wed, 9 Dec 2020 23:15:35 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Tudor Ambarus <tudor.ambarus@...rochip.com>,
        linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] spi: Limit the spi device max speed to controller's max
 speed

On Wed, Dec 09, 2020 at 07:54:20PM +0000, Mark Brown wrote:
> On Wed, Dec 09, 2020 at 10:46:36PM +0300, Serge Semin wrote:
> 
> > On Wed, Dec 09, 2020 at 07:35:14PM +0200, Tudor Ambarus wrote:
> 
> > > Make sure the max_speed_hz of spi_device does not override
> > > the max_speed_hz of controller.
> 
> > I have doubts that's right thing to do. It seems better to let
> > the controller driver to handle the speed clamping itself, while
> > to leave the SPI client device max_speed_hz field describing the
> > device speed capability. Moreover the SPI-transfers passed to the
> > controller will have a SPI-bus speed fixed in accordance with the
> > controller and client device capabilities anyway.
> > See the __spi_validate() method for details:
> > https://elixir.bootlin.com/linux/v5.10-rc7/source/drivers/spi/spi.c#L3570
> 

> Right, in general we aim to do this sort of fixup on the transfers
> and messages rather than the devices, I guess we might be missing
> validation in some of the flash acceleration paths or was this an issue
> seen through inspection?

In case of DW SPI driver we just make sure the SPI-client device
speed set in the max_speed_hz doesn't exceed the controller SPI-bus
clock frequency and clamp it if it does. So the driver is safe in that
matter.

-Sergey